Lithogenic hydrogen supports microbial primary production in subglacial and proglacial environments

Life in environments devoid of photosynthesis, such as on early Earth or in contemporary dark subsurface ecosystems, is supported by chemical energy. How, when, and where chemical nutrients released from the geosphere fuel chemosynthetic biospheres is fundamental to understanding the distribution and diversity of life, both today and in the geologic past. Hydrogen (H2) is a potent reductant that can be generated when water interacts with reactive components of mineral surfaces such as silicate radicals and ferrous iron. Such reactive mineral surfaces are continually generated by physical comminution of bedrock by glaciers. Here, we show that dissolved H2 concentrations in meltwaters from an iron and silicate mineral-rich basaltic glacial catchment were an order of magnitude higher than those from a carbonate-dominated catchment. Consistent with higher H2 abundance, sediment microbial communities from the basaltic catchment exhibited significantly shorter lag times and faster rates of net H2 oxidation and dark carbon dioxide (CO2) fixation than those from the carbonate catchment, indicating adaptation to use H2 as a reductant in basaltic catchments. An enrichment culture of basaltic sediments provided with H2, CO2, and ferric iron produced a chemolithoautotrophic population related to Rhodoferax ferrireducens with a metabolism previously thought to be restricted to (hyper)thermophiles and acidophiles. These findings point to the importance of physical and chemical weathering processes in generating nutrients that support chemosynthetic primary production. Furthermore, they show that differences in bedrock mineral composition can influence the supplies of nutrients like H2 and, in turn, the diversity, abundance, and activity of microbial inhabitants.

Dissolved inorganic carbon export from carbonate and silicate catchments estimated from carbonate chemistry and δ¹³C_DIC

This work presents a study of the dissolved inorganic carbon (DIC) exchange associated with groundwater discharge and stream flow from two upstream catchments with distinct basement lithologies (silicate vs. carbonate). The effects of catchment lithology were evident in the spring waters showing lower δ13CDIC and alkalinity (−16.2 ± 2.7 ‰ and 0.09 ± 0.03 meq l−1, respectively) in the silicate and higher values (−9.7 ± 1.5 ‰ and 2.0 ± 0.2 meq l−1) in the carbonate catchment. The streams exhibited relatively high δ13CDIC, −6.9 ± 1.6 ‰ and −7.8 ± 1.5 ‰, in silicate and carbonate catchments, respectively, indicating CO2 degassing during groundwater discharge and stream flow. The catchment lithology affected the pattern of DIC export. The CO2 degassing from stream and groundwater could be responsible for 8–55 % of the total DIC export in the silicate catchment, whereas the proportion is comparatively low (0.4–5.6 %) in the carbonate catchment. Therefore, the dynamic carbon exchange occurring at headwater regions and its possible variability with catchment lithology need to be examined for a more reliable carbon budget in river systems.

Dissolved inorganic carbon export from carbonate and silicate catchments estimated from carbonate chemistry and δ¹³C_DIC

We investigated dissolved inorganic carbon (DIC) exchange associated with groundwater discharge and stream flow from two upstream catchments with distinct basement lithology (silicate vs. carbonate). The effects of catchment lithology were evident in the spring waters showing lower δ13CDIC and alkalinity (−16.2 ± 2.7‰ and 0.09 ± 0.03 meq L−1, respectively) in the silicate and higher values (−9.7 ± 1.5‰ and 2.0 ± 0.2 meq L−1) in the carbonate catchment. The streams exhibited relatively high δ13CDIC values, −6.9 ± 1.6‰ and −7.8 ± 1.5‰, in silicate and carbonate catchments, respectively, indicating CO2 degassing during groundwater discharge and stream flow. The catchment lithology affected the pattern of DIC export. The CO2 degassing from stream and groundwater could be responsible for 8–55% of the total DIC export in the silicate catchment, whereas the proportion is comparatively low (0.4–5.6%) in the carbonate catchment. We emphasize the importance of dynamic carbon exchange occurring at headwater regions and its variability with catchment lithology for a more reliable carbon budget in river systems.